Coastal and anthropogenic heat impacts on PBL processes during extreme summer thunderstorm precipitation in New York City

Urban extreme precipitation is a crucial study topic due to its growing societal impact and limited understanding of its processes. This study examines the extreme summer precipitation event that occurred in coastal New York City (NYC) on July 17, 2018, using observations and simulations. Severe flooding was caused throughout the City by the thunderstorm event, which had a maximum rainfall of 44 mm and an Urban Heat Island (UHI) intensity of 8 °C. The study employs two high-resolution weather model simulations: one uses the Building Effect Parameterization (BEP), while the other incorporates with the BEP, Building Energy Model (BEM), an anthropogenic heat source, and a variable building drag coefficient. Forensics of the event show that increasing temperatures, and a strong UHI ahead of an approaching storm in the northwest initiated an urban storm cell near Bronx borough. The northwest-moving thunderstorm merged with the active urban cell and intensified the precipitation. Surface data indicates classic UHI signatures, such as pre-storm City-warming and post-storm cooling. The BEP + BEM model performed better than the BEP-only model by better simulating anthropogenic heat flux and projecting a greater UHI effect (3.8 vs. 2 °C). This improved simulation of anthropogenic heat produced high surface temperatures that strengthened low-level moisture convergence with the City, enhancing precipitation. The results highlight the key role played by anthropogenic heat in altering mesoscale atmospheric phenomena, strengthening storm intensity in coastal-urban environments. The findings of urban and anthropogenic processes on precipitation are transferable for improved urban-weather forecast and for planning resiliency of cities.